Forming inorganic fiber material containing cationic starch and colloidal silica



United States Patent FORD [ENG INORGANlC FHIER MATERIAL CUN- TAININGCATlQNlC STARCH AND COLLOXDAL SILICA Robert L. Brown, West Chester, Pa,and George W. Sears, Jr., Wilmington, DeL, assignors to E. ll. du Pontde Nemours and Company, Wilmington, Del., :1 corporation of Delaware NoDrawing. Filed Oct. 4, 1963, Ser. No. 313,743

t 1 Claim. (Cl. 162-455) This application is a continuation-in-part ofmy copcnding application Serial No. 129,226, filed August 4, 1961, nowabandoned.

This invention relates to heat-resistant compositions of inorganicfibers bonded together largely by colloidal silica and more particularlyis directed to a process for bonding together inorganic fibers withcolloidal silica whereby the resulting compositions obtain high andstable strength under prolonged exposure to high temperatures.

The compositions produced by the process of this invention comprisespecified inorganic fibers bonded in a three-dimensional network withcolloidal silica in the proportion of 2 to 20% by weight based on theweight of said fibers and a positively charged starch in the propor tionequal to 0.20 to 1.5% by weight based on the weight of said fibers.

These heat-resistant materials are prepared by dispersing in an aqueousmedium the inorganic fibers, adding thereto a positively charged starchin the proportion equal to 0.20 to 1.5% by weight based on the weight ofsaid fibers, then adding thereto colloidal silica in the proportion of 2to 20% by weight based on the weight of said fibers, acidification to pH3 to 5, removing said fibers from the aqueous medium and drying theresulting shaped product. it has been found that the acidification topii 3 to 5 markedly increases the strength of the resulting product overthat obtainable at the natural alkaline pH.

The product comprises articles madeup of the inorganic fibers bondedtogether largely by colloidal silica. The articles may take the form ofpapers, pressed board, mats, or formed objects such as tubes, rods,blocks, or other molded objects.

The inorganic fiber is selected from the group consisting of asbestos,paligorskite, attapulgite, quartz, glass, aluminosilicates, andpotassium titanate. The fibers prelerably are very fine so that thereare many points of contact. For this reason, forms of asebstos whosefibers are largely less than 0.05 micron in diameter will be strongerthan those containing much coarser fibers such as within the range of 1to microns.

The colloidal silica acting as a binder is present in an amount equal.to 2 to by weight based on the weight. of the fibers. However, in mostcases it is preferred to use 10 to 20% by weight. While less than 2%silica could be used, for most applications the bond strengths would beinsulficient to give adequate working strength. Since the silica isapplied to the fibers by exhaustion from solution, it is difiicult toincrease the concentration beyond 20% without excessive losses in thefiltrate.

Any of the silica sols of commerce may be used in this application.Although colloidal silica of any ultimate particle size will exhibitbinding action, it is preferred to use those sols containing particlesof average diameters in the range of 5 to millimicrons. Larger particlesgive less binding action per unit weight of silica due to the smallernumber of particles involved. For the same reason, it is preferred touse sols containing largely discrete particles rather than thosecontaining large aggregates.

In order to exhaust the colloidal silica onto the surice faces of thefibers, it is first necessary to add a positivel charged colloidalmaterial, allow it to absorb onto th surfaces of the fibers, and thensubsequently attract th negatively charged silica to itself by means ofits residue poistive charge. Especially desirable for this purpose arany of the positively charged starches commercially avail able, such asthe Cato starches of National Starch Prod ucts, lnc. Starches which havea positively electricz charge on the starch molecule can be prepared byreact ing ungelatinized starch with an ethcrifying agent cor taining atertiary amine radical as described in U.E 2,813,093, to C. G. Caldwelland O. B. Wurzburg.

The positively charged starches are especially useft in that in additionto their action as mordants, they als act as binders and impartconsiderable green strengt or strength prior to ignition to the articlebeing made The article is thus strengthened by the starch until bheating the silica bonding can be developed by dehydre tion and,crosslinking.

Organic binders, such as starch have been used here tofore with fiberssuch as asbestos, but such binders ar effective only at relatively lowtemperature and becom ineffective at temperatures well below the maximumus temperature of asbestos. Considerable smoking occur during use athigh temperatures, and this is undesirabl in many applications.Consequently, in the present ir vention, it is desirable to keep thepositively charge starch as low in concentration as possible. It is preferred to use no more than about 1.0% of the starc based on the totalweight of solids.

The inorganic fibers such as asbestos fibers are dis persed by forming adilute suspension of the fibers i water and stirring until anyaggregates of fibers ar swollen or broken apart. The upper limit ofconcentrz tion of fibers in the suspension is limited by the abilit tostir the slurry and will ordinarily be about 15% b weight. The lowerlimit is largely determined by ecr nomics, although exhaustion of thebinder onto the fiber is less efficient at greater dilution than in moreconcer trated slurry. Stirring may be done by a simple paddl stirrer ifthe fibers are fragile and long, or by a hight speed propeller if dangerof fiber breakage is small. preferred method is the use of a beater ascommon] used to disperse paper pulp.

A solution of the positive charged starch is added 1 the slurrycontaining the fibers as soon as the latter a1 dispersed. Theconcentration of this starch solution unimportant, but it is preferredto make it as concet trated as possible in order to reduce the volumerequire to a minimum. A concentration of 2 to 4% is preferre Thestarches are generally stirred into cold water at then the slurry heatednearly to the boiling point in 0rd to disperse and swell the granules.Extent of exhaustic of the positively-charged starch particles onto thefibe varies depending on the amount of starch, the difierent in charge,and the amount of fiber surface availabl There is added to the mixtureno more than 1.5% 1 weight of the starch, based on the weight of thefibe If exhaustion is only partial, addition of greater COl centrationsof the starch will largely be wasted, at moreover will tend to coagulatethe silica added next 211 consequently reduce its efficiency. After thestarch added, the slurry is stirred for at least 5 minutes to alloexhaustion to reach equilibrium.

For ultimate products in which appreciable organ content is undesirable,and which require high streng chiefly at elevated temperature, theamount of positive charged starch can be reduced to as low as 0.2 to 0.5based on the weight of the fibers provided that the Sllll content isincreased and the silica-fibers slurry is acic fied. Since one of theeffects of addition of the cation starch is to increase the bindingaction between fibers in the form during drying and before use atelevated temperature, reduction of this additive will result in a slightloss in green strength. This effect, however, disappears after heating,due to the increase in strength of the inorganic bond caused by mildheating.

As soon as the positively charged starch has been exhausted onto thefibers, the colloidal silica is added and stirred into the slurry. Aswith the starch, the mixing is not critical, and high speed agitation isnot essential. Thorough distribution through the slurry is of coursenecessary and should be accomplished within a minute or two. The silicamay be added at any convenient concentration, although it is preferredto add it at as high a concentration as possible to conserve storagespace. It will generally be added as received from the manufacturer.

Immediately following addition of the silica, the slurry is acidified toreduce the pH to the region 3 to 5. Although some binding effect isachieved by use of less than 10% silica based on the weight of fibers,it is much preferred to keep the silica content in the l'20% range. Itis recognized that some binding effect can be achieved in the absence ofacidification, although, the best strengths achieved are considerablyless than those obtained by the process of the present invention. In theabsence of acidification, it has been found that silica contents inexcess of are largely without effect.

Any acid can be used for this purpose, such as the mineral acids oracetic acid, although sulfuric acid is preferred for economic reasons.It is perfectly feasible to use concentrated sulfuric acid (e.g., 95%)on large-scale production without any predilution, Whereas on laboratoryscale, pH control of such a process would be difficult. In the lattercase, it is preferred to dilute the acid with at least 1 volume ofwater.

The acid is added so as to reduce the pH to the desired level within 1to 5 minutes. This pH level should then be held for at least 5 minutes.For fibers which are acid-soluble such as asbestos, further acidadditions must be made in order to maintain a low pH. Systems containingfibers which are inert to dilute acid, such as glass fibers, do notrequire further acid addition. For best results, the slurry should bestirred for a total of to 30 minutes after acid addition, allowing thepH to drift after the first 5 minutes after acid addition, although atleast as good results are obtained by holding the pH constant during theentire period.

The optimum pH to achieve maximum product strength is about 3.5. Productstrength falls off with further decrease in pH, but remains roughlyconstant in the region of pH 3.5 to 5, above which strength again fallsoff.

In order to form the product from the dilute slurry, the water must beremoved, as by filtration. In the making of paper, the slurry isfiltered either batchwise or continuously on a filter screen and thewater removed by pressing between filter cloths as by calendering. Ifother types of molded objects are desired, the slurry may be filtered toform a moist cake which can then be molded in forms, wrapped onmandrels, or the like.

The shaped form or wet paper is finally dried as by heating in an ovenor passing over heated rolls. Preferably, the final temperature of theproduct should be raised to 150 to 175 C. to insure complete drying.

Inorganic fibers have heretofore been bonded chiefly with organicbinders such as starch and casein. This is especially true in the fieldof asbestos papers and mats. Such binders, while strong at lowtemperature, are easily oxidized on heating, giving off smoke and fumesand rendering the product weak and substantially valueless for hightemperature application.

By using the process of the present invention, starchsilica binders areeffective to impart heretofore unobtainable strengths to inorganicfibers. Moreover, at elevated temperature the strength of the productdoes not deterio- Example 1 After slowly stirring a mixture of 7 gramsof grade 5R asbestos fibers, which means:

Grade Mesh Percent Percent fiber size fibers fibers passed collected and14.5 grams of grade 6D asbestos fibers, which means: 0

G rade Mesh Percent Percent: fiber size fibers fibers passed co ected 0in 1 liter of water, 13.5 milliliters of a 1.5% solution of positivelycharged starch is added. After stirring 10 minutes, 10 milliliters of asilica sol containing 10% silica-is added, and glacial acid is added toreduce the pH of the slurry to 2.5, and stirring is continued for 10minutes, and further acetic acid is added to maintain the pH at 2.5 fora total of 20 minutes. Three liters of water is added to the slurry andthe mixture dumped into the 8 inch by 8 inch head box of a hand-sheetpaper making apparatus. The water is filtered out as soon as currents inthe slurry subside, and the Wet sheet on the 100 mesh filter screen ispressed between blotters by light calendering, removed from the screen,and further pressed between fresh blotters, and then dried on a hotplate at 340 F. The tensile strength of this 20 mil paper is as follows:

Heated 1 hour, F None I (300 i 1,000

Example 2 To a slowly stirred mixture of 7 grams of grade 5R (as definedin Example 1) and 14.5 grams of grade 6D (as defined in Example 1)asbestos fibers in 1 liter of water, 13.5 milliliters of a 1.5% solutionof positively charged starch is added. After stirring for 10 minutes, 10milliliters of a silica sol containing 10% silica is added, and glacialacetic acid immediately added to reduce the pH within one minute to 3.5,and stirring is continued for 10 minutes, and further acetic acid isadded to maintain the pH at 3.5 for a total of 20 minutes. Three litersof water'is added to the slurry and the mixture dumped into the 8 inchby 8 inch head box of a hand-sheet paper making apparatus. The water isfiltered out as soon as currents in the slurry subside, and the wetsheet on the 100 mesh filter screen is pressed between blotters by lightcalendering, removed from the screen, and further pressed between freshblotters, and then dried on a hot plate at Heated 1 hour, F. I None I000 1,000

Lbs/in H 7. i 5.

Example 3 To a slowly stirred mixture of 7 grams of grade 5R (as definedin Example 1) and 14.5 grams of grade 6D (as defined in Example 1)asbestos fibers in 1 liter of water, 13.5 milliliters of a 1.5% solutionof positively charged starch is added. After stirring minutes,milliliters of silica sol containing 10% silica is added and glacialacetic acid immediately added to reduce the pH within 1 minute to 3.5and stirring is continued for 10 minutes. This pH is held constant byfurther acid addition for a total of 20 minutes. Three liters of wateris added to the slurry and the mixture dumped into the 8 inch by 8 inchhead box of a hand-sheet paper making apparatus. The water is filteredout as soon as currents in the slurry subside, and the wet sheet on the100 mesh filter screen is pressed between blotters by light calendering,removed from the screen, and further pressed between fresh blotters, andthen dried on a hot plate at 340 F. The tensile strength of this 20 milpaper is as follows:

Heated 1 hour, F None 000 1,000

Example 4 To a slowly stirred mixture of 7 grams of grade 5R (as definedin Example 1) and 14.5 grams of grade 6D (as defined in Example 1.)asbestos fibers in 1 liter of water, 13.5 milliliters of a 1.5% solutionof positively charged starch is added. After stirring 10 minutes, 20milliliters of silica sol containing 10% silica is added and glacialacetic acid immediately added to reduce the pH within 1 minute to 5.0and stirring is continued for 10 minutes. This pH is held constant byfurther acid addition for a total of 20 minutes. Three liters of wateris added to the slurry and the mixture dumped into the 8 inch by 8 inchhead box of a hand-sheet paper making apparatus. The water is filteredout as soon as currents in the slurry subside, and the wet sheet on the100 mesh filter screen is pressed between blotters by light calendering,removed from the screen and further pressed between fresh blotters, andthen dried on a hot plate at 340 F. The tensile strength of this 20 milpaper is as follows:

Heated 1 hour, F- l None 600 1 1,000

Lbs./lt1 "i 9.5 i 0.5 i 11.5

Example 5 milliliters of silica sol containing 10% silica is addec and50% sulfuric acid is used instead of glacial llCClit acid and the pH ofthe acidified slurry is adjusted to 3.7! and stirring is continued for10 minutes. The pH is helr constant by further acid addition for a totalof 20 minutes Three liters of water is added to the slurry and the mixture dumped into the 8 inch by 8 inch head box of hand-sheet papermaking apparatus. The water is filterei out as soon as currents in theslurry subside, and the we sheet is pressed between blotters by lightcalendering removed from the screen, and further pressed betweei freshblotters, and then dried on a hot plate at 340 F The tensile strength ofthis 20 mil paper is as follows:

To a two-liter slurry containing ,20 grams of fin aluminum silicatedispersed by beating 5 minutes in Hermann Disintegrator is added 13.5milliliters of a 1.5% solution of positively charged starch. The slurryis gentl stirred 10 minutes, and then 6 milliliters of silica sccontaining 30% silica is added. The slurry is immed: ately acidified topH 3.6 with 25% sulfuric acid an stirring continued for 20 minutesduring which time th pH stayed constant. The slurry is diluted to 4liter: then dumped into the 8 inch by 8 inch head box of hand-sheetpaper making apparatus. The water is fi tered out as soon as currents inthe slurry subside, an the wet sheet on the 100 mesh filter screen ispressed be tween blotters by light calendering, removed from th screen,and further pressed between fresh blotters, an then dried on a hot plateat 340 F. The tensile strengt of this sheet is as follows:

Lbs./i1 As is 1. 15 min. at 900 C. 2.

We claim:

In a process for making a heat-resistant material is dispersing in anaqueous medium an inorganic fibc selected from the group consisting ofasbestos, paligor: kite, attapulgite, quartz, glass, aluminosilicatesand Pt tassium titanate, adding positively charged starch an colloidalsilica, removing said fibers from the aqueot medium, and drying theresulting shaped product, tl: steps comprising (1) first adding thepositively charge starch to the said dispersion in the proportion of 0.2t 1.5% by weight based on the weight of said fibers, (2 then adding thecolloidal silica in the proportion of 1 to 20% by weight based on theweight of the fibers, an (3) then adding acid in an amount sutficient tolow the pH of the dispersion to the range of 3 to 5.

References Cited by the Examiner UNITED STATES PATENTS DONALL H.SYLVESTER, Primary Examiner.

